0413

GLACIAL-EUSTATIC CONTROL OF COALBED METHANE RESERVOIR DISTRIBUTION (POTTSVILLE FORMATION; LOWER PENNSYLVANIAN) IN THE BLACK WARRIOR BASIN OF ALABAMA

Jack C. Pashin and Dorothy E. Raymond Geological Survey of Alabama, P.O. Box 869999, Tuscaloosa, AL 35486

ABSTRACT

Subsurface maps and stratigraphic cross sections of Pennsylvanian-age strata in the Black Warrior basin provide new evidence for the origin of late Paleozoic depositional cycles and the stratigraphic controls on the distribution of coalbed methane reservoirs. Coal beds are concentrated in a series of 4th- order parasequences, or cyclothems, which are bounded by regionally extensive marine flooding surfaces. Each cyclothem represents about 0.4 my, which is equivalent to the long orbital eccentricity period. The cyclothems studied each contain three subordinate marine flooding surfaces defining 5th- order parasequences, which apparently are products of relative sea-level changes associated with the short eccentricity period (0.1 my). The first 5th-order parasequence in each cyclothem tends to be dominated by progradational deltaic deposits. The second and third 5th-order parasequences contain a higher proportion of aggradational deposits and include incised valley fills as deep as 100 ft overlain by widespread reservoir coal beds as thick as 10 ft. The fourth parasequence commonly contains transgressive tidal deposits. Although cyclothems provide an obvious basis for stratigraphic subdivision of Pennsylvanian strata in the Black Warrior basin, internal facies relationships suggest that major sea-level change was most effective in the short eccentricity band, as was the case during Pleistocene glaciation. Accordingly, rising base level in the second and third 5th-order parasequences of most cyclothems set the stage for widespread peat accumulation and preservation of the thickest and most widespread coalbed methane reservoirs in the Black Warrior basin.

INTRODUCTION

The geometry and distribution of coalbed methane reservoirs varies significantly among sedimentary basins. In Cretaceous and Tertiary strata of the western USA, for example, coalbed methane is produced from a single, thick (~10-100 ft) coal bed or from multiple thick seams within a single coal zone [e.g., 1, 2]. In Pennsylvanian strata of the eastern USA, by comparison, reservoir coal beds are relatively thin (~1-12 ft) and are in multiple zones distributed through a thick stratigraphic section [3-6]. In the upper Pottsville Formation of the Black Warrior basin, coal beds are in places dispersed through more than 4,000 ft of section, and gas has been produced from more than 20 coal beds in some wells (figs. 1, 2). Distribution of Pennsylvanian coal beds through a thick stratigraphic section is largely the product of repetitive marine-terrestrial depositional cyclicity, and the origin of this cyclicity has been debated since the historic publications on cyclothems by Weller [7] and Wanless and Shepard [8]. Recent debate has focused on the tectonic and paleoclimatic implications of this cyclicity [e.g., 9-11], which is thought to be a product of basin subsidence associated with assembly of the Pangaean supercontinent, as well as eustasy driven by Gondwanan glaciation [12-14]. In the Black Warrior basin, close well spacing and a thick stratigraphic section facilitate investigation of stratal geometry and patterns of coal distribution in multiple depositional cycles [15-18]. This paper presents detailed subsurface maps and stratigraphic cross sections that provide new evidence for the origin of Pennsylvanian cycles and the geologic controls on the distribution of coalbed methane reservoirs. 2 COALBED METHANE RESERVOIR DISTRIBUTION IN THE BLACK WARRIOR BASIN

INDEX MAP White Oak Creek C Short Creek A'

Blue Creek Oak Grove

Pleasant Grove Alabama

Brookwood EXPLANATION Deerlick Creek Coalbed gas well APPALACHIAN Field boundary B Robinson's Holt Bend FRONTAL STRUCTURESC Stratigraphic cross section Peterson C' Little Sandy Taylor Creek C' Black Warrior basin Creek Cedar Cove Thornton Thrust fault Creek Anticline Syncline Little Buck Creek Big Sandy Creek Moundville B' 5 0510 15 km A 5 0 5 10 mi

Figure 1. Index map showing locations of coalbed methane fields in the Black Warrior basin, well control, and lines of cross section used in this study.

Methods

More than 5,000 gamma-density logs and long cores were described and correlated to identify and classify stratigraphic markers, such as marine flooding surfaces; to identify and define sedimentary structures and lithofacies; and to determine the geometry and extent of sandstone bodies and reservoir coal beds (fig. 1). A series of stratigraphic cross sections was then constructed showing the stratigraphic architecture of the target coal zones (plates 1-3). Selected cross sections are presented in this paper, and a complete set is in Pashin et al. [19]. The cross sections depict gamma-density logs, major depositional cycles and marine flooding surfaces, subordinate parasequences, coal beds, and major sandstone units. Coal beds and associated organic-rich shale beds were classified according to thickness and density-log signature into primary resource targets, secondary resource targets, and thin marker beds. Primary resource targets are coal beds thicker than 2 ft and typically have a blocky log signature, whereas secondary resource targets are between 1 and 2 ft thick and have a pronounced spike signature (< 1.5 g/cc). Thin marker beds include coal and organic shale markers that are less than 1 ft thick and have a subdued spike signature (> 1.5 g/cc). After the coal beds were correlated, parasequences subordinate to the major depositional cycles were defined by correlating regionally extensive shale and coal markers. Stratigraphic and well-location data from all wells were compiled into a spreadsheet to facilitate subsurface mapping. Well locations were computed from surveyed line calls using the Wellbase module PASHIN AND RAYMOND 3

SOMED Permit 8911-C Duncanville Curry 12-10-370 Core Cedar Cove field

Brookwood GR Density coal zone FS

500 ft Utley coal zone EXPLANATION

Rock types

1000 FS Mudstone

Gwin Interbedded mudstone coal zone and sandstone Sandstone (litharenite) FS Coal 1500 Cobb coal zone Structures

Marine fossils

FS Burrows Plant fossils, 2000 Pratt root traces coal zone Pebbles

Maximum FS flooding surface FS 2500 Gillespy coal zone

FS

3000 Mary Lee coal zone

FS Ream coal zone 3500 FS

Black Creek coal zone

Figure 2. Core log and geophysical well log of the upper Pottsville Formation in Cedar Cove Field showing coal zones and 4th-order maximum flooding surfaces bounding cyclothems [after 18]. 4 COALBED METHANE RESERVOIR DISTRIBUTION IN THE BLACK WARRIOR BASIN of the Geographix Exploration System. Stratigraphic data include the depth of each cycle boundary and net coal thickness in each coal zone. Coal thickness was determined using high-resolution density logs, which typically have a scale of 1 inch equals 25 ft. Coal beds thinner than 1 ft generally are not logged accurately, are seldom completed for gas production, and were thus excluded from the thickness determination. Maps of coal thickness were gridded and contoured using a minimum curvature algorithm in the Isomap module of Geographix. Net coal isolith maps were made for each target coal zone, and a net coal isolith map was made for the Black Creek through Brookwood coal zones.

Background

Coalbed methane reserves in the Black Warrior basin are concentrated in the upper Pottsville Formation (Lower Pennsylvanian; Langsettian), which comprises nine major coal zones and is dominated by siliciclastic rocks (fig. 2). More than 5,000 coalbed methane wells have been drilled, and more than 4,000 wells are currently producing. Cumulative gas production now exceeds 1.5 Tcf, and annual production has exceeded 110 Bcf since the mid 1990s. Coalbed methane resources are estimated to be between 10 and 20 Tcf [20, 21], and reserves are estimated to be between 2.5 and 4.6 Tcf [4, 22, 23]. The Black Warrior basin is a late Paleozoic foreland basin that formed at the juncture of the Appalachian and Ouachita orogenic belts [24-26]. The coalbed methane fields are in the eastern part of the basin adjacent to the frontal structures of the Appalachian orogen (fig. 1). Carboniferous strata in the Black Warrior basin thicken southwest, indicating that subsidence was dominated by thrust and sediment loading in the Ouachita orogen [27]. In the coalbed methane fields, however, upper Pottsville strata thicken southeastward, indicating superposition of an Appalachian flexural moat on the larger Ouachita foreland basin [15, 27, 28] (fig. 3). The area containing the thickest section in the coalbed methane fields is called the Moundville-Cedar Cove depocenter [19]. Net completable coal thickness in the upper Pottsville ranges from less than 10 ft in the northwestern part of the coalbed methane fairway to more than 70 ft in parts of the Moundville-Cedar Cove depocenter (fig. 4). Stratigraphic grouping of Pottsville coal beds was observed by McCalley [29], and intercalated marine and terrestrial facies were recognized by Butts [30]. From the 1960s through the 1980s, the Alabama Pottsville formed a basis for facies models of Appalachian coal-bearing strata [25, 26, 31, 32]. During extensive natural gas exploration, regionally extensive depositional cycles were identified and mapped [3, 15, 28]. Recent workers [e.g., 18, 33, 34] have interpreted the upper Pottsville cycles as flooding-surface- bounded depositional units, or parasequences [see 35, 36]. The base of each parasequence is marked by a condensed section with abundant marine fossils. Above the condensed section is a thick interval (100-300 ft) of marine shale that coarsens upward into fluvial-deltaic sandstone. At the top of each parasequence is a lithologically heterogeneous coal zone that accumulated in a spectrum of marginal marine and terrestrial environments. Pashin [15, 16, 34] interpreted these parasequences to have been deposited at a frequency no greater than 0.4 my, which is equivalent to the Milankovitch long eccentricity period that is thought to have driven the formation of Pennsylvanian cyclothems [11, 13]. Accordingly, the major Pottsville parasequences analyzed in this study can be classified as cyclothems, or 4th-order parasequences.

POTTSVILLE PARASEQUENCES

Pottsville cyclothems are readily recognized in gamma-density logs (fig. 2; plates 1-3). Shale has radioactivity higher than 100 gammas, whereas sandstone has radioactivity lower than 100 gammas. The maximum flooding surface bounding each cyclothem is typically marked by elevated radioactivity at the bottom of a thick shale interval, and these surfaces can be traced throughout the study area. The basal shale units have a serrate log pattern and coarsen upward into sandstone, which is typical of progradational deposits. Framework sandstone has a blocky or fining-upward log signature, which is characteristic of aggradational deposits. Coal beds and associated organic-rich shale units are identified by low density, and coal beds thicker than 1 ft have low radioactivity similar to sandstone. The cyclothems contain subordinate progradational intervals (fig. 2; plates 1-3). These intervals are best developed in the Gillespy through Gwin coal zones, and some minor flooding surfaces at the tops of these intervals can be correlated across the study area (plates 2, 3). These surfaces define the boundaries of high-frequency parasequences. Each cyclothem studied comprises four subordinate parasequences, suggesting sedimentation in the short eccentricity Milankovitch band (~0.1 my). Hence, PASHIN AND RAYMOND 5

1500

EXPLANATION 1500 1,000-1,500 ft 2000 1,500-2,000 ft 2,000-2,500 ft > 2,500 ft

2500 5 0510 15 km 5 0 5 10 mi 2000

Moundville-Cedar Cove depocenter 2500

Figure 3. Isopach map showing thickness of the upper Pottsville Formation from the top of the Black Creek coal zone to the top of the Gwin coal zone. the subordinate depositional units are interpreted as 5th-order parasequences. These parasequences are color-coded blue, green, yellow, and brown on the basis of their position within each cyclothem (plates 1- 3). The progradational signature of the subordinate parasequences is most readily apparent in the blue and green color-coded intervals. In the green through brown parasequences, however, the progradational signature is commonly obscured or supplanted by aggradational sandstone and shale units. The tops of widespread coal beds commonly mark or correlate with marine flooding surfaces [36, 37] (plates 1-3), and flooding surfaces above regionally extensive coal beds in the study area were verified with core and outcrop data. The 5th-order parasequences contain three dominant lithofacies: (1) progradational shale and sandstone, (2) aggradational sandstone and shale, and (3) coal.

Progradational Shale and Sandstone

Progradational shale is very dark gray to dark medium gray and is silty or sandy. Marine fossils are most abundant in the lower parts of the shale units and include brachiopods, molluscs, echinoderms, and solitary corals [30, 38]. The shale typically contains horizontal burrows. Sandstone forms laminae to thick beds and is medium gray to light gray. The thickness, frequency, and grain size of these beds increases upsection. Sandstone beds in the lower parts of the progradational units are typically graded, and higher in section, graded sandstone beds can be traced laterally into clinoform foreset beds [39]. Near the tops of these intervals, crossbeds and scour fills are common. In the brown parasequences, graded bedding is absent, and marine body fossils are rare. These intervals are dominated by wavy-, flaser-, and lenticular- 6 COALBED METHANE RESERVOIR DISTRIBUTION IN THE BLACK WARRIOR BASIN

EXPLANATION > 70 ft 30-40 ft 60-70 ft 20-30 ft 50-60 ft 10-20 ft 40-50 ft < 10 ft Contour interval = 10 ft

5 0510 15 km

5 0 5 10 mi

Moundville-Cedar Cove depocenter

Figure 4. Net coal isolith map of the upper Pottsville Formation. bedded shale and sandstone containing diverse trace fossils and are truncated by regionally extensive ravinements that underlie the condensed section of the next cyclothem [18, 40]. The deltaic character of the progradational shale and sandstone units has long been recognized [16, 18, 31, 33, 39]. In the blue and green parasequences, the basal parts of the progradational shale units are typical of prodeltaic deposits, whereas graded sandstone beds are typical of distal delta front deposits. Clinoform foreset bedding is common in distal to medial distributary mouth bars, whereas crossbeds and scour fills are characteristic of proximal mouth bars. Significantly, 5th-order flooding surfaces define northwest-dipping clinoforms in the lower parts of the Gillespy through Gwin coal zones (plates 2, 3). Wavy-, flaser-, and lenticular-bedded shale and sandstone in the brown parasequences, by contrast, are characteristic of tidal deposits [41, 42]. Pashin [17, 18] interpreted these strata as retrograding delta-destructive and estuarine facies that formed during the early stages of 4th-order marine transgressions. The ravinements capping the brown parasequences have been interpreted as transgressive surfaces that formed by shoreface erosion during regional inundation [18, 40].

Aggradational Sandstone and Shale

Aggradational sandstone bodies are abundant in the middle and upper parts of each cyclothem, and the thickness and continuity of these bodies is variable (fig. 2; plates 1-3). The sandstone is compositionally immature, is fine- to coarse-grained, and locally includes conglomerate. Isolith maps indicate that sandstone is thickest in the Moundville-Cedar Cove depocenter [3, 15]. The sandstone units have sharp bases, fine upward, and have gradational tops. Sedimentary structures include crossbeds, PASHIN AND RAYMOND 7 current-ripple cross laminae, horizontal laminae, scour-and-fill structures, and lateral accretion surfaces [3, 42, 43]. Biogenic structures range from plant fossils to invertebrate traces and, rarely, calcareous shells. Single-story sandstone bodies are most common in the blue and brown parasequences, and multistory sandstone bodies are most common in the green and yellow parasequences. Shale is commonly interbedded with sandstone, and bedding styles range from pinstripe bedding to wavy, flaser, and lenticular bedding; rooted mudstone also is common. In places, thick, graded sandstone beds are common in the shale units, as are muddy channel fills. Fossils in the aggradational shale units range from erect plant assemblages to the burrows and trackways of invertebrates and vertebrates, as well as sparse brachiopod and mollusc shells [38]. The sandstone bodies have been interpreted to represent a spectrum of contributive, transitive, and distributive fluvial systems, as well as tidal channel systems [17, 18, 31, 43, 44]. Concentration of compositionally immature sandstone in the Moundville-Cedar Cove depocenter suggests a source in the Appalachian orogen [3, 15]. Some of the sandstone units in the coalbed methane fields, such as in the green parasequence of the Gwin coal zone (plate 3, C-C’), appear to have cut as deeply as 100 ft into the underlying shale units and can thus be interpreted as incised valley fills. The associated shale units appear to have been deposited in a broad array of interchannel environments and include overbank, crevasse-splay, bayfill, backswamp, and tidal-flat deposits [31, 39, 42, 43].

Coal

Pottsville coal is bright-banded and composed of vitrinite (70-95%) with lesser inertinite (5-30%) and liptinite (0-8%) [45]. Ash content is typically 5 to 19% and is dominated by detrital clay and quartz, and total sulfur content is typically 0.8 to 3.0% [19]. Net coal thickness in the upper Pottsville increases from less than 10 ft in the western coalbed methane fields to more than 70 ft in parts of the Moundville-Cedar Cove depocenter (fig. 4). In the Mary Lee and Pratt coal zones, net coal thickness exceeds 10 ft in several areas along the southeast margin of the Black Warrior basin (figs. 3, 5). In the Gwin and Utley zones, net coal thickness exceeds 5 ft mainly in the Moundville-Cedar Cove depocenter (figs. 3, 6). Coal beds are most numerous in the Moundville-Cedar Cove depocenter (plates 1-3); they are rare in the blue parasequences and are most common in the green and yellow parasequences. Coal beds are sparse in brown parasequences, save for those above the Pratt coal bed (plate 3). Most upper Pottsville coal beds can be correlated for large distances (plates 1-3). Several thin marker beds and secondary resource beds appear to be truncated by aggradational sandstone bodies, and many appear to be in facies relationship with sandstone. In contrast, major resource beds and the correlative secondary resource beds and thin markers can be traced across large parts of the study area. Most of these beds have been named (see plates 1-3) and are the principal mining and degasification targets in the Black Warrior basin. Most named beds are at or are a short distance below flooding surfaces defining the boundaries of 5th-order parasequences. Care must be taken, however, when associating coal beds with flooding surfaces. For example, it is tempting to place a parasequence boundary at the top of the upper Cobb bed based on log signature and the thickness of adjacent siliciclastic strata (plate 3). However, no correlative flooding surface can be identified in cross-section C-C’. The vast majority of the coal beds are geometrically simple; that is, a single bed can be traced for large distances without splitting (plates 1-3). Where multiple beds merge, as is the case in the Mary Lee, Blue Creek, and Gwin beds of Big Sandy Creek and Moundville fields (plate 1, A-A’; plate 3, A-A’), individual coal beds tend to retain identity in density logs, and high-resolution well logs can be used for bench-scale correlation. A prominent example of bed splitting is in the Pratt seam, where a thick bed with multiple benches splits basinward toward a large multistory sandstone body (plate 2, C-C’). Channel-fill coal bodies are in the Blue Creek coal of Oak Grove Field (plate 1, A-A’). The channels are up to 60 ft deep, truncate Jagger coal, and form a dendritic network [17]. The coal thickens to more than 9 ft in the channels and is a primary target of longwall coal mining and mine-related degasification activities. The Alabama Pottsville is part of the Euramerican coal belt, which formed as a widespread system of equatorial peat swamps during Pennsylvanian time [46]. High detrital ash content indicates that peat accumulated mainly in low-lying swamps, although some low-ash coal has been attributed to doming [47]. Sulfur content is higher than 2% in coal with marine roof strata [48, 49], and the broad range of sulfur content in the Pottsville reflects preservation of coal below marine through terrestrial roof facies. The variable thickness and extent of Pottsville coal beds (plates 1-3; figs. 2-6) further points to diverse origins. Many of the thin, discontinuous beds arguably are the products of localized swamps that 8 COALBED METHANE RESERVOIR DISTRIBUTION IN THE BLACK WARRIOR BASIN

A. Mary Lee coal zone

EXPLANATION < 1 ft 1-5 ft 5-10 ft 10-15 ft 15-20 ft Contour interval = 5 ft

5 0 5 10 15 km 5 0 5 10 mi

Moundville-Cedar Cove depocenter

B. Pratt coal zone

EXPLANATION

< 1 ft 10-15 ft 1-5 ft 15-20 ft 5-10 ft 20-25 ft Contour interval = 5 ft

5 0 5 10 15 km 5 0 5 10 mi

Moundville-Cedar Cove depocenter

Figure 5. Net coal isolith maps of the Mary Lee and Pratt coal zones. PASHIN AND RAYMOND 9

A. Gwin coal zone

EXPLANATION < 1 ft 10-15 ft 1-5 ft 15-20 ft 5-10 ft 20-25 ft Contour interval = 5 ft

5 0 5 10 15 km 5 0 5 10 mi

Moundville-Cedar Cove depocenter

B. Utley coal zone

EXPLANATION < 1 ft 10-15 ft 1-5 ft 15-20 ft 5-10 ft 20-25 ft

Contour interval = 5 ft

5 0 5 10 15 km 5 0 5 10 mi

Moundville-Cedar Cove depocenter

Figure 6. Net coal isolith maps of the Gwin and Utley coal zones. 10 COALBED METHANE RESERVOIR DISTRIBUTION IN THE BLACK WARRIOR BASIN were prone to erosion or pass laterally into contemporaneous siliciclastic sediment. At the other extreme, regionally extensive coal beds provide evidence for widespread paludification of the coastal plain [see 50]. Regional aggradation appears to have facilitated paludification. For example, Pashin [17] interpreted the channel fill coal of the Blue Creek bed (plate 1, A-A’) to indicate infilling of an aggraded, dendritic paleovalley system with peat. Other widespread coal beds, like the Cobb and Gwin beds, accumulated after channel belts had aggraded and filled completely with siliciclastic sediment (plate 3). Interestingly, the Mary Lee-Blue Creek channel-fill coal is in the interior of a 5th-order parasequence (plate 1), whereas most other regionally extensive coal beds are at or a short distance below parasequence-bounding flooding surfaces (plates 1-3). As such, aggradation of sandy channel systems and accumulation of the Gwin bed was a prelude to a widespread marine flooding event. By contrast, progressive splitting of Pratt coal and interfingering with contemporaneous siliciclastic sediment (plate 2, C-C’) suggests that some major swamp systems persisted through 5th-order flooding events.

PALEOCLIMATIC AND TECTONIC INTERPRETATIONS

Pennsylvanian cyclothems are commonly attributed to 4th-order changes of relative sea level in the long eccentricity band, although the mechanisms driving this cyclicity remain controversial [9, 11]. Berger and Loutre [51] showed that the intensity of insolation varied in concert with long eccentricity during Pleistocene glaciation (fig. 7A). However, Rial [52] demonstrated that glacial ice volume was regulated by short eccentricity and that long eccentricity modulated higher Milankovitch frequencies (fig. 7B). Regardless of the cause, 4th-order depositional cycles provide an obvious basis for stratigraphic subdivision of the upper Pottsville (fig. 2; plates 1-3). The classic cyclothems of the North American midcontinent are much thinner than those in the Black Warrior basin, and correlative strata in the Appalachian basin contain major stratigraphic discontinuities [53]. Thus, interpretations of cyclicity in Pennsylvanian strata have been hampered by low stratigraphic resolution. The exceptional thickness of upper Pottsville cyclothems, coupled with close well spacing in the coalbed methane fields, facilitates the recognition of 5th-order parasequences (plates 1-3; fig. 8) and thus provides new insight into the depositional, climatic, and tectonic processes that operated during Pennsylvanian time. Taken together, the blue through green parasequences typically define progradational parasequence sets (fig. 8). The brown parasequences can represent continued progradational stacking, as is the case in the Gillespy and Pratt coal zones (plate 2). However, abundant marginal-marine deposits and limited sandstone and coal resources indicate retrogradation, as is the case in the brown parasequences of the Mary Lee and Gwin coal zones (figs. 2, 8; plates 1, 3). The blue parasequences record major episodes of deltaic progradation and basin filling and thus constitute 4th-order highstand deposits (plates 1-3). Although the green and yellow parasequences can also contain progradational deltaic deposits, especially in the northwestern parts of the study area, these parasequences contain complex multistory sandstone bodies and some incised valley fills, suggesting that lowstand surfaces of erosion are present and that fluvial sedimentation had some sensitivity to base- level changes in the short eccentricity band. However, complex facies relationships within the 5th-order parasequences make lowstand surfaces difficult to trace. The incised valley fills in the green parasequence of the Gwin coal zone (plate 3, C-C’) suggest that sea-level drops were about 100 ft, which is consistent with other estimates of glacial eustasy during the Early Pennsylvanian [54]. Although the stratigraphic expression of the brown parasequences is variable, retrogradation in some cyclothems indicates deposition during 4th-order marine transgressions. Coal beds appear to be the products of interwoven autogenic and allogenic processes. The high abundance of coal beds in the Moundville-Cedar Cove depocenter reflects increased tectonic subsidence and persistence of terrestrial sedimentation in areas proximal to the sediment source. Within the depocenter coal beds, such as those above the Pratt seam (plate 3), can be too numerous to be explained solely by high-frequency Milankovitch processes, specifically obliquity (~40 ky) and precession (~20 ky). Autogenic processes that may have operated include channel avulsion and autocompaction of peat, which can cause rapid shifts in the locations of flood basins and swamps. Although more coal beds have been completed in the depocenter than in any other part of the Black Warrior basin, increased coal resources do not necessarily translate to large gas reserves or better well performance [3, 19]. Total effective subsidence rates in the Moundville-Cedar Cove depocenter during upper Pottsville deposition at times exceeded 1,500 ft/my [34, 55], so sediment compaction combined with tectonic processes driven by thrust and sediment loading may have also played an important role in the PASHIN AND RAYMOND 11

A. ~400,000 yr insolation cycles 520

500

480 N lat) ° 460 at 65 2 440 Insolation

420 (watts/m

400

380 0 400,000 800,000 Time before present (yr)

B. ~100,000 and ~40,000 yr ice volume cycles

Short eccentricity dominant Obliquity dominant High ODP site 806

Low

ODP site 849 High

Low

ODP site 677 Global ice volume High

Low

0 400,000 800,000 1,200,000 1,600,000 Time before present (yr) Ice volume calculated from oxygen isotope record Modulated short eccentricity signal

Figure 7. Insolation and ice-volume curves showing the impact of the long (~0.4 my) and short (~0.1 my) eccentricity signals on climate during Pleistocene time [after 51, 52). These same factors may have regulated glacial eustasy and the development of cyclothems during the Pennsylvanian. preservation of coal and the intervening siliciclastic rocks. Gastaldo et al. [56] argued that catastrophic subsidence events explain the preservation of standing forests in which erect plants extend upward through more than 20 ft of aggradational shale. Rapid subsidence events also may help explain the alternation of geometrically simple coal beds with siliciclastic strata in the interiors of many 5th-order parasequences. Furthermore, rapid subsidence apparently kept upper Pottsville fluvial systems close enough to grade to favor the development of single-story and poorly confined multistory sandstone bodies over well-confined paleovalley fills in most parasequences. Although many processes affected peat accumulation, base level changes appear to have played a fundamental role in the formation of the thickest and most widespread coal beds, which are the principal targets for gas production, as well as underground and surface mining. The common association between widespread coal beds and 5th-order flooding events suggests that regional aggradation driven by high- 12 COALBED METHANE RESERVOIR DISTRIBUTION IN THE BLACK WARRIOR BASIN

0.4 my

Single-story channel fill

0.3 my

0.2 my Multistory channel fill 0.1 my

Incised valley fill

0.0 my EXPLANATION

Primary resource target Cycle-bounding flooding surface Coal Secondary resource target Minor flooding surface Aggradational sandstone (color-coded by parasequence) Shells and burrows

Plants Interbedded shale and sandstone (color-coded by parasequence)

Figure 8. Idealized stratigraphic model showing facies relationships and flooding surfaces in upper Pottsville cyclothems of the Black Warrior coalbed methane fields.

frequency glacial eustasy helped curb siliciclastic sediment flux and subdue topography, thereby laying the groundwork for the growth of regionally extensive peat swamps. Although aggradation prior to marine flooding favored the development of major swamps, the thick valley-fill coal of the Mary Lee-Blue Creek subzone (plate 1) suggests that subsidence coupled with the earliest stages of eustatic sea-level rise could raise water tables enough to promote the formation of a major resource bed in the interior of a 5th- order parasequence.

SUMMARY AND CONCLUSIONS

Coalbed methane reserves in the upper Pottsville Formation are distributed among multiple 4th-order depositional cycles, or cyclothems, that in places span more than 4,000 ft of section. Upper Pottsville strata thicken southeastward into a prominent foreland flexure adjacent to the Appalachian orogen. Coal beds and resources are most abundant in this flexure, and isolith maps indicate that the impact of tectonic subsidence on coal thickness patterns increased through time during Pottsville sedimentation. Each cyclothem studied comprises four 5th-order parasequences, suggesting control of sedimentation by glacial eustasy. The first of these parasequences is dominated by deltaic deposits and contains limited coal resources. The second and third parasequences contain thick, aggradational sandstone, including incised valley fills. The thickest and most widespread coal beds are within these parasequences, and together, the first three parasequences exhibit progradational stacking. The character of the fourth parasequence is variable, indicating continued progradation or retrogradation. This parasequence commonly contains transgressive tidal facies and generally lacks major coal resources. Most upper Pottsville coal beds are geometrically simple; that is, individual beds can be traced without splitting. However, some beds include channel-fill coal bodies or progressive bed splits adjacent to contemporaneous siliciclastic facies. Regionally extensive coal beds are associated with 5th-order PASHIN AND RAYMOND 13 base-level rises. These base-level rises facilitated aggradation of fluvial systems, smoothing of topography, and reduction of sediment flux, thereby setting the stage for paludification of the coastal plain and accumulation of the most important reservoir coal beds and mining targets in the basin.

ACKNOWLEDGMENTS

This report was prepared with the support of the U.S. Department of Energy under Award no. DE- FC26-00NT40927. However, any opinions, findings, conclusions, or recommendations expressed herein are those of the authors and do not necessarily reflect the views of the DOE. Marcella R. McIntyre helped correlate major flooding surfaces and tabulated large volumes of data from geophysical well logs. Plates 1-3 are designed to be plotted on ARCH D-size paper.

REFERENCES CITED

[1] Flores, R. M., 1993, Coal-bed and related depositional environments in methane gas-producing sequences: AAPG Studies in Geology, v. 38, p. 13-37. [2] Ayers, W. B., Jr., 2002, Coalbed gas systems, resources, and production and a review of contrasting cases from the San Juan and Powder River Basins: AAPG Bulletin, v. 86, p. 1853-1890. [3] Pashin, J. C., Ward, W. E., II, Winston, R. B., Chandler, R. V., Bolin, D. E., Richter, K. E., Osborne, W. E., and Sarnecki, J. C., 1991, Regional analysis of the Black Creek-Cobb coalbed-methane target interval, Black Warrior basin, Alabama: Alabama Geological Survey Bulletin 145, 127 p. [4] Lyons, P. C., 1997, Central-northern Appalachian coalbed methane flow grows: Oil & Gas Journal, July 7, 1997, p. 76-79. [5] Markowski, A. K., 1998, Coalbed methane resource potential and current prospects in Pennsylvania: International Journal of Coal Geology, v. 38, p. 137-159. [6] Nolde, J. E., and Spears, D., 1998, A preliminary assessment of in place coalbed methane resources in the Virginia portion of the central Appalachian Basin: International Journal of Coal Geology, v. 38, p. 115-136. [7] Weller, S., 1930, Cyclic sedimentation of the Pennsylvanian Period and its significance: Journal of Geology, v. 38, p. 97-135. [8] Wanless, H. R., and Shepard, F. P., 1936, Sea level and climatic changes related to late Paleozoic cycles: GSA Bulletin, v. 47, p. 1177-1206. [9] Klein, G. D., and Willard, D. A., 1989, Origin of the Pennsylvanian coal-bearing cyclothems of North America: Geology, v, 17, p. 152-155. [10] Heckel, P.H., 1977, Origin of phosphatic black shale facies in Pennsylvanian cyclothems of midcontinent North America: AAPG Bulletin, v. 61, p. 1045-1068. [11] Heckel, P.H., 1986, Sea-level curve for Pennsylvanian eustatic transgressive-regressive depositional cycles along midcontinent outcrop belt, North America: Geology, v. 14, p. 330-334. [12] Ross, C. A., and Ross, J. R. P., 1988, Late Paleozoic transgressive-regressive deposition: SEPM Special Publication 42, p. 227-247. [13] Heckel, P.H., 1994, Evaluation of evidence for glacio-eustatic control over marine Pennsylvanian cyclothems in North America and consideration of possible tectonic effects: SEPM Concepts in Sedimentology and Paleontology 4, p. 65-87. [14] Klein, G.D., 1994, Depth determination and quantitative distinction of the influence of tectonic subsidence and climate on changing sea level during deposition of midcontinent Pennsylvanian cyclothems: SEPM Concepts in Sedimentology and Paleontology 4, p. 35-50. [15] Pashin, J. C., 1994, Flexurally influenced eustatic cycles in the Pottsville Formation (Lower Pennsylvanian), Black Warrior basin, Alabama: SEPM Concepts in Sedimentology and Paleontology, v. 4, p. 89-105. [16] Pashin, J. C., 1994, Cycles and stacking patterns in Carboniferous rocks of the Black Warrior foreland basin: GCAGS Transactions, v. 44, p. 555-563. [17] Pashin, J. C., 1994, Coal-body geometry and synsedimentary detachment folding in Oak Grove coalbed-methane field, Black Warrior basin, Alabama: AAPG Bulletin, v. 78, p. 960-980. [18] Pashin, J. C., 1998, Stratigraphy and structure of coalbed methane reservoirs in the United States: An overview: International Journal of Coal Geology, v. 35, p. 207-238. 14 COALBED METHANE RESERVOIR DISTRIBUTION IN THE BLACK WARRIOR BASIN

[19] Pashin, J. C., Carroll, R. E., Groshong, R. H., Jr., Raymond, D. E., McIntyre, M. R., and Payton, J. W., 2004, Geologic screening criteria for sequestration of CO2 in coal: Quantifying potential of the Black Warrior coalbed methane fairway, Alabama: Final Technical Report, U.S. Department of Energy, contract DE-FC26-00NT40927, 254 p. [20] Hewitt, J. L., 1984, Geologic overview, coal, and coalbed methane resources of the Warrior basin—Alabama and Mississippi: AAPG Studies in Geology 17, p. 73-104. [21] McFall, K. S., Wicks, D. E., and Kuuskraa, V. A., 1986, A geological assessment of natural gas from coal seams in the Warrior basin, Alabama - topical report: GRI contract no. 5084-214-1066, 80 p. [22] Rice, D. D., 1995, Geologic framework and description of coal-bed gas plays: USGS Digital Data Series DDS-30, 103 p. [23] Hatch, J. R., Pawlewicz, M. J., Charpentier, R. E., Cook, T. A., Crovelli, R. A., Klett, T. R., Pollastro, R. M., and Schenk, C. J., 2003, Assessment of undiscovered oil and gas resources of the Black Warrior Basin province, 2002; U.S. Geological Survey Fact Sheet FS-038-03, 2 p. [24] Mellen, F. F., 1947, Black Warrior basin, Alabama and Mississippi: AAPG Bulletin, v. 31, p. 1801- 1816. [25] Thomas, W. A., 1974, Converging clastic wedges in the Mississippian of Alabama: GSA Special Paper 148, p. 187-207. [26] Thomas, W. A., 1988, The Black Warrior basin: GSA, The geology of North America, v. D-2, p. 471- 492. [27] Whiting, B. M., and Thomas, W. A., 1994, Three-dimensional controls on subsidence of a foreland basin associated with a thrust-belt recess, Black Warrior basin, Alabama and Mississippi: Geology, v. 22, p. 727-730. [28] Sestak, H. M., 1984, Stratigraphy and depositional environments of the Pennsylvanian Pottsville Formation in the Black Warrior basin, Alabama and Mississippi (thesis): University of Alabama, 184 p. [29] McCalley, H., 1900, Report on the Warrior coal basin: Alabama Geological Survey Special Report 10, 327 p. [30] Butts, C., 1926, The Paleozoic rocks: Alabama Geological Survey Special Report 14, p. 41-230. [31] Ferm, J. C., Ehrlich, R., and Neathery, T. L., 1967, A field guide to Carboniferous detrital rocks in northern Alabama: GSA 1967 Coal Division Field Trip Guidebook, 101 p. [32] Ferm, J. C., and Weisenfluh, G. A., 1989, Evolution of some depositional models in Late Carboniferous rocks of the Appalachian coal fields: International Journal of Coal Geology, v. 12, p. 259-292. [33] Gastaldo, R. A., Demko, T. M., and Liu, Y., 1993, Application of sequence and genetic stratigraphic concepts to Carboniferous coal-bearing strata: An example from the Black Warrior Basin, USA: Geologische Rundschau, v. 82, p. 212-226. [34] Pashin, J. C., 2004, Cyclothems of the Black Warrior basin, USA: eustatic snapshots of foreland basin tectonism: AAPG Studies in Geology 51, in press. [35] Vail, P. R., 1987, Seismic stratigraphy interpretation procedure: AAPG Studies in Geology 27, 1:1-10. [36] Van Wagoner, J. C., Mitchum, R. M., Campion, K. M., and Rahmanian, V. D., 1991, Siliciclastic sequence stratigraphy in well logs, cores, and outcrops: AAPG Methods in Exploration 7, 55 p. [37] Martino, R. L., 2004, Sequence stratigraphy of the Glenshaw Formation (Late Pennsylvanian) in the southern Dunkard basin: AAPG Studies in Geology 51, in press. [38] Gastaldo, R. A., Demko, T. M., and Liu, Y. eds., 1990, Carboniferous coastal environments and paleocommunities of the Mary Lee coal zone, Marion and Walker Counties, Alabama: Alabama Geological Survey Guidebook Series VI, 139 p. [39] Rheams, L. J., and Benson, D. J., 1982, Depositional setting of the Pottsville Formation in the Black Warrior basin: Alabama Geological Society 19th Annual Field Trip Guidebook, 94 p. [40] Liu, Y., and Gastaldo, R. A., 1992, Characteristics of a Pennsylvanian ravinement surface: Sedimentary Geology, v. 77, p. 197-213. [41] Pashin, J. C., Carroll, R. E., Barnett, R. L., and Beg, M. A., 1995, Geology and coal resources of the Cahaba coal field: Alabama Geological Survey Bulletin 163, 49 p. [42] Demko, T. M., and Gastaldo, R. A., 1996, Eustatic and autocyclic influences on deposition of the lower Pennsylvanian Mary Lee coal zone, Warrior Basin, Alabama: International Journal of Coal Geology, v. 31, p. 3-19. PASHIN AND RAYMOND 15

[43] Demko, T. M., and Gastaldo, R. A., 1992, Paludal environments of the Lower Mary Lee coal zone, Pottsville Formation, Alabama: Stacked clastic swamps and peat mires: International Journal of Coal Geology, v. 20, p. 23-47. [44] Horsey, C. A., 1981, Depositional environments of the Pennsylvanian Pottsville Formation in the Black Warrior basin of Alabama: Journal of Sedimentary Petrology, v. 51, p. 799-806. [45] Carroll, R. E., and Pashin, J. C., 2003, Relationship of sorption capacity to coal quality: CO2 sequestration potential of coalbed methane reservoirs in the Black Warrior basin: 2003 International Coalbed Methane Symposium Proceedings, Paper 0317, 11 p. [46] Phillips, T.L., and Peppers, R.A., 1984, Changing patterns of Pennsylvanian coal-swamp vegetation and implications of climatic control: International Journal of Coal Geology, v. 3, p. 205-255. [47] Eble, C. F., Gastaldo, R. A., Demko, T. M., and Liu, Y., 1994, Coal compositional changes along a swamp interior to swamp margin transect in the Mary Lee Coal bed, Warrior basin, Alabama, U.S.A: International Journal of Coal Geology, v. 26, p. 43-62. [48] Williams, E. G., and Keith, M. L., 1963, Relationship between sulfur in coals and the occurrence of marine roof beds: Economic Geology, v. 58, p. 720-729. [49] Casagrande, D. J., 1987, Sulphur in peat and coal: London Geological Society Special Publication 32, p. 87-105. [50] Diessel, C. F. K., 1998, Sequence stratigraphy applied to two coal seams: Two case histories: SEPM Special Publication 59, p. 151-173. [51] Berger, A. L., and Loutre, M. F., 1991, Insolation values for the climate of the last 10 million years: Quaternary Science Review, v. 10, p. 297-317. [52] Rial, J. A., 1999, Pacemaking the ice ages by frequency modulation of Earth's orbital eccentricity: Science, v. 285, 564-568. [53] Heckel, P. H., Gibling, M. R., and King, N. R., 1998, Stratigraphic model for glacial-eustatic Pennsylvanian cyclothems in highstand nearshore detrital regimes: Journal of Geology, v. 106, p. 373-383. [54] Maynard, J. R., and Leeder, M. R., 1992, On the periodicity and magnitude of Late Carboniferous glacio-eustatic sea-level changes: Journal of the Geological Society (London), v. 149, p. 303-311. [55] Thomas, W. A., Ferrill, B. A., Allen, J. L., Osborne, W. E., and Leverett, D. E., 1991, Synorogenic clastic-wedge stratigraphy and subsidence history of the Cahaba synclinorium and the Black Warrior foreland basin: Alabama Geological Society 28th Annual Field Trip Guidebook, p. 37-39. [56] Gastaldo, R. A., Stevanovic-Walls, I., and Ware, W. N., 2004, In situ, erect forests are evidence for large-magnitude, coseismic base-level changes within Pennsylvanian cyclothems of the Black Warrior basin, USA: AAPG Studies in Geology, 51, in press. 2004 INTERNATIONAL COALBED METHANE SYMPOSIUM PROCEEDINGS PLATE 1

A Moundville Field Big Sandy Creek Field Cedar Cove Field Brookwood Field Oak Grove Field A' Southwest T T Northeast 8326 7836 9286 9324 6877 7438 9541 6602 9200 9645 9236 9239 10220 8309 6911 6671 7596 10246 10284 4615 4159 3756 9703 10104 6100 7203 6361 8993 5136 5341 10342 6483 7263 7595 7363 7340

50

0 New Castle New Castle Mary Lee 50 New Castle Mary Lee Mary Lee Mary Lee coal zone Blue Creek Jagger 100 New Castle Blue Creek Jagger Jagger Blue Creek e t 150 New Castle Mary Lee

200 a l e i n f Blue Creek

a l s c 250 Blue Creek Jagger c t e r

V 300

350

400

450

500

0 2 4 6 8 10 km

0 1 2 3 4 5 6 mi

Little Sandy T Cedar Cove B Robinson's Bend Field Moundville Field B' C Blue Creek Field Deerlick Creek Field Holt Field Peterson Field C' Northwest Creek Field Southeast Northwest Field T Southeast 7779 8205 8522 8781 9338 7321 7693 8104 9014 7086 7046 7130 6643 7495 9463 9324 8946 9446 9527 8874 9170 9745 8215 8314 7957 7979 7399 7402 6195 4308 4006 4166 9918 8915 9692 8449 9812 6911 8019 7390 50 50 0 0 New Castle

50 e t e t 50 New Castle New Castle Mary Lee 100 Blue Creek Mary Lee 100 a l e i n f a l e i n f New Castle Jagger Blue Creek Mary Lee 150 Mary Lee Mary Lee

Jagger New Castle a l s c

a l s c 150

Blue Creek c coal zone c Jagger coal zone t

t Jagger

200 e r e r V 200 V Blue Creek 250 Jagger Jagger 250 300 300 0 2 4 6 8 10 km 350 0 1 2 3 4 5 6 mi 0 2 4 6 8 10 km

0 1 2 3 4 5 6 mi EXPLANATION

Cycle-bounding flooding surface 6424 Well permit number Minor flooding surface Gamma Density ray Primary resource bed Gamma-density logs Coal Secondary resource bed Thin marker bed STRATIGRAPHIC CROSS SECTIONS OF THE MARY LEE COAL ZONE Aggradational sandstone T Tie log IN THE BLACK WARRIOR COALBED METHANE FIELDS, ALABAMA (color-coded by parasequence)

By Interbedded shale and sandstone Jack C. Pashin and Dorothy E. Raymond (color-coded by parasequence) 2004 2004 INTERNATIONAL COALBED METHANE SYMPOSIUM PLATE 2

A Moundville Field Big Sandy Creek Field Cedar Cove Field Brookwood Field Oak Grove Field A' Southwest T T Northeast 8262 8136 9286 8816 6877 7438 9541 6602 9200 9645 9236 9239 10220 8309 6911 6671 7596 10276 4159 3756 9703 10104 6100 7203 6361 5780 5136 5341 10342 6483 7263 7595 7363 7340

50

0

50

100

150 Pratt e t

200 Pratt Pratt a l e i n f 250 Pratt a l s c American American c t American Pratt e r 300 Pratt V coal American 350 Curry American zone Curry 400 Curry Curry Curry 450 Gillespy Gillespy Gillespy Gillespy Gillespy Gillespy 500 coal zone 600

700

800

900

1000

0 2 4 6 8 10 km

0 1 2 3 4 5 6 mi

Little Sandy T Cedar Cove Field B Robinson's Bend Field Moundville Field B' C Blue Creek Field Deerlick Creek Field Holt Field Peterson Field C' Northwest Creek Field Southeast Northwest T Southeast 7779 7967 8205 8522 8412 9224 8781 9338 7321 7693 8104 9337 7086 7046 7130 6643 7495 9463 8816 8946 9446 9527 9117 9170 9745 8215 8314 7957 7979 7399 7402 5903 6195 4308 4006 4166 9918 8915 9692 8449 9812 6911 8019 7390

50

0

50

50 100 Pratt Pratt coal zone 0 coal zone 150 e t

50 200 Pratt Pratt Pratt a l e i n f 100 250 Pratt a l s c c

t Pratt 150 Pratt American e t Pratt e r 300 American V American 200 350 a l e i n f American 250 Curry a l s c 400 Curry c t Curry e r 300

V 450 Curry Curry Gillespy Gillespy Gillespy 350 Curry Gillespy 500 coal zone

400 Gillespy Gillespy 600 Gillespy Gillespy 450 coal zone 700

500 800

600 900

700 1000 800 0 2 4 6 8 10 km

900 0 1 2 3 4 5 6 mi 1000 EXPLANATION 0 2 4 6 8 10 km Cycle-bounding 0 1 2 3 4 5 6 mi flooding surface 6424 Well permit number Minor flooding surface Gamma Density ray Primary resource bed Gamma-density logs Coal Secondary resource bed STRATIGRAPHIC CROSS SECTIONS OF THE GILLESPY, CURRY, AND PRATT COAL ZONES Thin marker bed Aggradational sandstone T Tie log IN THE BLACK WARRIOR COALBED METHANE FIELDS, ALABAMA (color-coded by parasequence) By Jack C. Pashin and Dorothy E. Raymond Interbedded shale and sandstone 2004 (color-coded by parasequence) 2004 INTERNATIONAL COALBED METHANE SYMPOSIUM PLATE 3

A A' Moundville Field Big Sandy Creek Field Cedar Cove Field Brookwood Field Oak Grove Field Southwest T T Northeast 8262 8136 9286 9324 6877 7438 9541 6602 9200 9645 9236 9239 10220 8309 9344 6671 7596 10276 10123 5494 9703 10104 6100 7203 6347 8993 5136 6483 7363 7340

500

450

400

350 Gwin 300 coal zone Gwin 250 Gwin

200 Thompson Mill Gwin Thompson Mill 150 Thompson Mill Gwin Thompson Mill 100 e t 50

a l e i n f 0 Upper Cobb Upper Cobb Upper Cobb a l s c 50 Cobb c Cobb Cobb t Cobb Cobb Upper Cobb e r Cobb V 100 coal Thomas zone Thomas 150 Thomas 200

250

300

350

400

450

500

550

600

0 2 4 6 8 10 km

0 1 2 3 4 5 6 mi

B Little Sandy T B' C C' Robinson's Bend Field Moundville Field Blue Creek Field Deerlick Creek Field Holt Field Peterson Field Cedar Cove Northwest Creek Field Southeast Northwest Field T Southeast 7779 8138 8205 8522 8660 8781 9338 7321 7692 8804 9245 7086 7046 7130 6643 7495 9463 9324 8946 9446 9527 8874 9170 9745 8215 8314 7957 7744 7400 7499 6365 6195 4308 4003 4166 9918 8915 9692 8449 9812 9344 8019 7390

Gwin 250 Gwin Gwin 300 Gwin coal coal Gwin 200 zone 250 Thompson zone Gwin Gwin Mill 150 Thompson Mill 200 Thompson Mill Thompson Mill 100 150

e t 50 100 0 a l e i n f e t Upper 50 Cobb 50 Cobb Cobb Cobb a l s c Cobb c

t Cobb a l e i n f 0 coal e r 100 Upper Cobb V zone a l s c 50 Upper Cobb Cobb Thomas c Upper t Cobb Cobb Cobb 150 e r Cobb V 100 coal zone 200 150 Thomas 250 200 300 250 350 300 0 2 4 6 8 10 km 400 350 0 1 2 3 4 5 6 mi

400 0 2 4 6 8 10 km EXPLANATION 0 1 2 3 4 5 6 mi Cycle-bounding flooding surface 6424 Well permit number Minor flooding surface Gamma Density ray Primary resource bed Gamma-density logs Coal Secondary resource bed Thin marker bed STRATIGRAPHIC CROSS SECTIONS OF THE COBB AND GWIN COAL ZONES Aggradational sandstone T Tie log IN THE BLACK WARRIOR COALBED METHANE FIELDS, ALABAMA (color-coded by parasequence)

By Interbedded shale and sandstone Jack C. Pashin and Dorothy E. Raymond (color-coded by parasequence) 2004